Decreasing or eliminating the concentration and/or function of intracellular proteins post--translationally may obviate the need for or offer an alternative to the use of antisense constructs, as well as the use of 'knockout' technology. Moreover, if within an individual experiment one could eliminate an intracellular protein concentration and thereafter revert the cell to normalcy, a specific function of a given protein could be elegantly studied. The current project strives to accomplish just such an experimental system by altering the binding sites of heavy chain antibody antigen recognition sequences (HcFv) by random substitution within the nucleotide coding frames of the antigen determining regions (DR1/DR3). The recombinant constructs define novel binding proteins and also permit insertion of the recombinant(s) of interest into mammalian shuttle vectors having inducible promoters, such that a binding protein can be temporarily expressed intracellularly. A recombinant phage library expressing 10(12) different binding proteins on their surface as fusion proteins (fM 13p*-HcFv) have been constructed. A set of newly engineered recombinant proteins were isolated from the random library that specifically binds and eliminates the activity of glucose-6-phosphate dehydrogenase (G6PD). We chose one of the set of binding proteins and inserted its gene into a mammalian cell. Expression of the binding protein results in ablation of intracellular G6PD activity. Further, once induction has ceased, G6PD activity returns fully, i.e., a mutant and its internal revertant within the same cell has been constructed. The method is being adapted for use in binding oncogenic and viral proteins.